This invention relates generally to electrical power delivery systems for offshore and sub-sea electrical loads via a direct current (DC) transmission bus. The receiving end and sending end of the DC transmission bus each comprise modular stacked power converters that are symmetrical in structure. The receiving end converters are reconfigurable based on site expansion requirements and on load types and configurations.
There is a growing industry need to deliver power, more effectively with lower cost and higher reliability/maintainability, efficiency and power density, from onshore or offshore platforms to electric loads at seabed or remote offshore locations, or vice versa in a reverse power flow direction for offshore power generation tie-back. This growing need is driven by electrification trends in various applications, such as the subsea processing for oil and gas industry and offshore wind power production.
Specifically for subsea processing for oil and gas industry, the trends are (1) more electric loads, such as electric drives and motors driving pumps and compressors for subsea processing, subsea control and communication electronics, electric pipeline heating, power supply for separator/coalescers; (2) higher power—from kilowatts to approaching 100 MW range per project; (3) longer distance—from tens of kilometers to 100˜600 km; and (4) deeper water depth—from 1 km to 3 km.
To serve a large number of electric loads distributed in a region at subsea and offshore locations over a short or long distance, electric power typically needs to be transmitted from onshore or offshore platform power sources to a subsea or offshore power substation via a power transmission bus, and then further distributed to those electric loads via a power distribution bus. In some cases, newly discovered oil and gas reserves with electric loads need to be tied back to an adjacent already established power generation/transmission/distribution infrastructure.
System architectures to transmit and distribute power effectively to those subsea and offshore loads is very important—from a choice of alternate current (AC) or direct current (DC) power transmission and distribution, to selection of voltage level for transmission and distribution, to a system topological architecture. They significantly affect system cost, reliability/maintainability, system complexity, efficiency and power density. For example, offshore or subsea cables for power transmission typically form a dominant portion of overall system cost. Compared with three-phase AC power transmission, DC power transmission reduces the number and weight of cables, thus potentially reducing material and installation costs. A higher voltage for power transmission/distribution would reduce cable losses, and therefore result in higher efficiency and less cable costs. However, the electric loads may need medium voltage or low voltage, and an additional power conversion stage would be needed to convert the transmission/distribution voltage to the requisite load voltage level. An optimal system architecture would result in significantly less system complexity and cost. Subsea connectors, such as wet-mate and dry-mate connectors, and fault tolerant operation capability by bypassing faulty elements have a great impact on system reliability and maintainability. System architectures that allow a reduction in the number of subsea connectors and that provide fault tolerant operation capability are of utmost importance for long time reliable operation for subsea and offshore applications.
Three-phase 50/60 Hz AC power transmission and distribution is a mature technology. However, it has inherent limitations for long distance and high power subsea or offshore applications, or even for applications with short distance but with limited capacity margins of the power source. Due to the cable capacitance, a significant amount of reactive power needs to be supplied from the power source and carried by the cable, in addition to the active power needed by the loads. This results in higher cable losses, higher current ratings and larger and more costly cables, and higher voltage losses along the cable. These issues are exacerbated for long distance and high power transmission for oil and gas subsea projects. Even for short distance power transmission/distribution, these issues still exist for applications with a limited capacity margin of the power source. For example, for electric loads that are tied back to an existing power infrastructure on a offshore platform with limited capacity margin, a relatively large amount of reactive power may trigger power system stability issues or exceed current rating limits of the power source.
The limitations of 50/60 Hz AC power transmission and distribution may be alleviated by reducing the AC frequency, to for example 16⅔ Hz, thus reducing the amount of reactive power under the same cable capacitance. However, this solution is at the expense of proportionally increased size of magnetic components, such as transformers. At high power levels, the size and weight penalty would be excessive.
Direct current (DC) power transmission and distribution can fundamentally overcome the cable capacitance and reactive power issue for power delivery; and high voltage would further reduce losses for power transmission and distribution. Existing high voltage direct current technology uses simple 2-level circuit topology and relies on series connections of a large number of specially power switches, such as press-pack IGBTs and thyristors, to provide high voltage capability for power conversion. Due to high voltage switching with 2-level circuits, large filters are needed to smooth out the input and output. Those special power switches (valves) and large filters would make existing high voltage direct current technology an expensive and bulky solution for subsea applications.
Alternative high voltage or medium voltage direct current technology forms DC transmission or distribution bus stacking using a number of modular power converter building blocks. Since those building blocks can be made the same as those in other standard drive applications, the stacked modular DC technology offers potentially much lower cost and higher reliability. Furthermore, harmonic cancellation on the AC side can be achieved by control means for those modular converters such that filters can be significantly smaller at lower cost.
There is a need to address system architectures based on the DC transmission bus formed by modular stacked converters for power transmission and distribution serving multiple electric loads. The key objectives are to achieve optimum power delivery systems with low system cost and complexity, high system reliability/maintainability, high efficiency and power density. The targets are for applications where single or plural electric loads need to be served at subsea or offshore locations, with long or short distance, and with high or low power.